SPEERMINT D. Malas, Ed.
Internet-Draft CableLabs
Intended status: Informational J. Livingood, Ed.
Expires: August 22, 2011 Comcast
February 18, 2011
Session PEERing for Multimedia INTerconnect Architecturedraft-ietf-speermint-architecture-19
Abstract
This document defines a peering architecture for the Session
Initiation Protocol (SIP), its functional components and interfaces.
It also describes the components and the steps necessary to establish
a session between two SIP Service Provider (SSP) peering domains.
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Internet-Draft SPEERMINT Peering Architecture February 20111. Introduction
This document defines a reference peering architecture for the
Session Initiation Protocol (SIP)[RFC3261], it's functional
components and interfaces, in the context of session peering for
multimedia interconnects. In this process, we define the peering
reference architecture, its functional components, and peering
interface functions from the perspective of a SIP Service Provider's
(SSP) [RFC5486] network. Thus, it also describes the components and
the steps necessary to establish a session between two SSP peering
domains.
An SSP may also be referred to as an Internet Telephony Service
Provider (ITSP). While the terms ITSP and SSP are frequently used
interchangeably, this document and other subsequent SIP peering-
related documents should use the term SSP. SSP more accurately
depicts the use of SIP as the underlying layer 5 signaling protocol.
This architecture enables the interconnection of two SSPs in layer 5
peering, as defined in the SIP-based session peering requirements
[I-D.ietf-speermint-requirements].
Layer 3 peering is outside the scope of this document. Hence, the
figures in this document do not show routers so that the focus is on
layer 5 protocol aspects.
This document uses terminology defined in the Session Peering for
Multimedia Interconnect (SPEERMINT) Terminology document [RFC5486].
Apart from normative references included herein, readers may also
find [I-D.ietf-speermint-voip-consolidated-usecases] informative.
2. New Terminology
[RFC5486] is a key reference for the majority of the SPEERMINT-
related terminology used in this document. However, some additional
new terms are used here as follows in this section.
2.1. Session Border Controller (SBC)
A Session Border Controller (SBC) is referred to in Section 5. An
SBC can contain a Signaling Function (SF), Signaling Path Border
Element (SBE) and Data Path Border Element (DBE), and may perform the
Look-Up Function (LUF) and Location Routing Function (LRF) functions,
as described in Section 3. Whether the SBC performs one or more of
these functions is generally speaking dependent upon how a SIP
Service Provider (SSP) configures such a network element. In
addition, requirements for an SBC can be found in [RFC5853].
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Internet-Draft SPEERMINT Peering Architecture February 20112.2. Carrier-of-Record
A carrier-of-record, as used in Section 6.1.2.2, is defined in
[RFC5067]. That document describes the term to refer to the entity
having discretion over the domain and zone content and acting as the
registrant for a telephone number, as represented in ENUM. This can
be:
o the Service Provider to which the E.164 number was allocated for
end user assignment, whether by the National Regulatory Authority
(NRA) or the International Telecommunication Union (ITU), for
instance, a code under "International Networks" (+882) or
"Universal Personal Telecommunications (UPT)" (+878) or,
o if the number is ported, the service provider to which the number
was ported, or
o where numbers are assigned directly to end users, the service
provider that the end user number assignee has chosen to provide a
Public Switched Telephone Network/Public Land Mobile Network
(PSTN/ PLMN) point-of-interconnect for the number.
It is understood that the definition of carrier-of-record within a
given jurisdiction is subject to modification by national
authorities.
3. Reference Architecture
The following figure depicts the architecture and logical functions
that form peering between two SSPs.
For further details on the elements and functions described in this
figure, please refer to [RFC5486]. The following terms, which appear
in Figure 1, which are documented in [RFC5486] are reproduced here
for simplicity.
- Data Path Border Element (DBE): A data path border element (DBE) is
located on the administrative border of a domain through which flows
the media associated with an inter-domain session. It typically
provides media-related functions such as deep packet inspection and
modification, media relay, and firewall-traversal support. The DBE
may be controlled by the SBE.
- E.164 Number Mapping (ENUM): See [RFC3761].
- Fully Qualified Domain Name (FQDN): See Section 5.1 of [RFC1035].
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- Location Routing Function (LRF): The Location Routing Function
(LRF) determines for the target domain of a given request the
location of the SF in that domain, and optionally develops other SED
required to route the request to that domain. An example of the LRF
may be applied to either example in Section 4.3.3 of [RFC5486]. Once
the ENUM response or SIP 302 redirect is received with the
destination's SIP URI, the LRF must derive the destination peer's SF
from the FQDN in the domain portion of the URI. In some cases, some
entity (usually a 3rd party or federation) provides peering
assistance to the originating SSP by providing this function. The
assisting entity may provide information relating to direct (Section4.2.1 of [RFC5486]) or indirect (Section 4.2.2 of [RFC5486]) peering
as necessary.
- Look-Up Function (LUF): The Look-Up Function (LUF) determines for a
given request the target domain to which the request should be
routed. An example of an LUF is an ENUM [4] look-up or a SIP INVITE
request to a SIP proxy providing redirect responses for peers. In
some cases, some entity (usually a 3rd party or federation) provides
peering assistance to the originating SSP by providing this function.
The assisting entity may provide information relating to direct
(Section 4.2.1 of [RFC5486]) or indirect (Section 4.2.2 of [RFC5486])
peering as necessary.
- Real-Time Transport Protocol (RTP): See [RFC3550].
- Session Initiation Protocol (SIP): See [RFC3261].
- Signaling Path Border Element (SBE): A signaling path border
element (SBE) is located on the administrative border of a domain
through which inter-domain session layer messages will flow. It
typically provides signaling functions such as protocol inter-working
(for example, H.323 to SIP), identity and topology hiding, and
Session Admission Control for a domain.
- Signaling Function (SF): The Signaling Function (SF) performs
routing of SIP requests for establishing and maintaining calls, and
to assist in the discovery or exchange of parameters to be used by
the Media Function (MF). The SF is a capability of SIP processing
elements such as SIP proxies, SBEs, and user agents.
- SIP Service Provider (SSP): A SIP Service Provider (SSP) is an
entity that provides session services utilizing SIP signaling to its
customers. In the event that the SSP is also a function of the SP,
it may also provide media streams to its customers. Such an SSP may
additionally be peered with other SSPs. An SSP may also interconnect
with the PSTN.
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The originating or indirect SSP would perform steps 1-4, the target
SSP would perform steps 4, and either one can perform step 5.
In the case the target SSP changes, then steps 1-4 would be repeated.
This is reflected in Figure 1 that shows the target SSP with its own
peering functions.
5. Relationships Between Functions/Elements
Please also refer to Figure 1.
o An SBE can contain a Signaling Function (SF).
o An SF can perform a Look-Up Function (LUF) and Location Routing
Function (LRF).
o As an additional consideration, a Session Border Controller, can
contain an SF, SBE and DBE, and may act as both an LUF and LRF.
o The following functions may communicate as follows in an example
SSP network, depending upon various real-world implementations:
* SF may communicate with LUF, LRF, SBE and SF
* LUF may communicate with SF and SBE
* LRF may communicate with SF and SBE
6. Recommended SSP Procedures
This section describes the functions in more detail and provides some
recommendations on the role they would play in a SIP call in a Layer
5 peering scenario.
Some of the information in the section is taken from
[I-D.ietf-speermint-requirements] and is included here for continuity
purposes. It is also important to refer to Section 3.2 of
[I-D.ietf-speermint-voipthreats], particularly with respect to the
use of IPSec and TLS.
6.1. Originating or Indirect SSP Procedures
This section describes the procedures of the originating or indirect
SSP.
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Internet-Draft SPEERMINT Peering Architecture February 20116.1.1. The Look-Up Function (LUF)
The purpose of the LUF is to determine the SF of the target domain of
a given request and optionally to develop Session Establishment Data.
It is important to note that the LUF may utilize the public e164.arpa
ENUM root, as well as one or more private roots. When private roots
are used specialized routing rules may be implemented, and these
rules may vary depending upon whether an originating or indirect SSP
is querying the LUF.
6.1.1.1. Target Address Analysis
When the originating (or indirect) SSP receives a request to
communicate, it analyzes the target URI to determine whether the call
needs to be routed internal or external to its network. The analysis
method is internal to the SSP; thus, outside the scope of SPEERMINT.
If the target address does not represent a resource inside the
originating (or indirect) SSP's administrative domain or federation
of domains, then the originating (or indirect) SSP performs a Lookup
Function (LUF) to determine a target address, and then it resolves
the call routing data by using the Location routing Function (LRF).
For example, if the request to communicate is for an im: or pres: URI
type [RFC3861] [RFC3953], the originating (or indirect) SSP follows
the procedures in [RFC3861]. If the highest priority supported URI
scheme is sip: or sips: the originating (or indirect) SSP skips to
SIP DNS resolution in Section 5.1.3. Likewise, if the target address
is already a sip: or sips: URI in an external domain, the originating
(or indirect) SSP skips to SIP DNS resolution in Section 6.1.2.1.
This may be the case, to use one example, with
"sips:bob@biloxi.example.com".
If the target address corresponds to a specific E.164 address, the
SSP may need to perform some form of number plan mapping according to
local policy. For example, in the United States, a dial string
beginning "011 44" could be converted to "+44", or in the United
Kingdom "00 1" could be converted to "+1". Once the SSP has an E.164
address, it can use ENUM.
6.1.1.2. ENUM Lookup
If an external E.164 address is the target, the originating (or
indirect) SSP consults the public "User ENUM" rooted at e164.arpa,
according to the procedures described in [RFC3761]. The SSP must
query for the "E2U+sip" enumservice as described in [RFC3764], but
may check for other enumservices. The originating (or indirect) SSP
may consult a cache or alternate representation of the ENUM data
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rather than actual DNS queries. Also, the SSP may skip actual DNS
queries if the originating (or indirect) SSP is sure that the target
address country code is not represented in e164.arpa.
If an im: or pres: URI is chosen based on an "E2U+im" [RFC3861] or
"E2U+pres" [RFC3953] enumserver, the SSP follows the procedures for
resolving these URIs to URIs for specific protocols such as SIP or
XMPP as described in the previous section.
The NAPTR response to the ENUM lookup may be a SIP AoR (such as
"sips:bob@example.com") or SIP URI (such as
"sips:bob@sbe1.biloxi.example.com"). In the case of when a SIP URI
is returned, the originating (or indirect) SSP has sufficient routing
information to locate the target SSP. In the case of when a SIP AoR
is returned, the SF then uses the LRF to determine the URI for more
explicitly locating the target SSP.
6.1.2. Location Routing Function (LRF)
The LRF of an originating (or indirect) SSP analyzes target address
and target domain identified by the LUF, and discovers the next hop
signaling function (SF) in a peering relationship. The resource to
determine the SF of the target domain might be provided by a third-
party as in the assisted-peering case. The following sections define
mechanisms which may be used by the LRF. These are not in any
particular order and, importantly, not all of them have to be used.
6.1.2.1. DNS Resolution
The originating (or indirect) SSP uses the procedures in Section 4 of
[RFC3263] to determine how to contact the receiving SSP. To
summarize the [RFC3263] procedure: unless these are explicitly
encoded in the target URI, a transport is chosen using NAPTR records,
a port is chosen using SRV records, and an address is chosen using A
or AAAA records.
When communicating with another SSP, entities compliant to this
document should select a TLS-protected transport for communication
from the originating (or indirect) SSP to the receiving SSP if
available, as described further in Section 6.2.1.
6.1.2.2. Routing Table
If there are no End User ENUM records and the originating (or
indirect) SSP cannot discover the carrier-of-record or if the
originating (or indirect) SSP cannot reach the carrier-of-record via
SIP peering, the originating (or indirect) SSP may deliver the call
to the PSTN or reject it. Note that the originating (or indirect)
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SSP may forward the call to another SSP for PSTN gateway termination
by prior arrangement using the local SIP proxy routing table.
If so, the originating (or indirect) SSP rewrites the Request-URI to
address the gateway resource in the target SSP's domain and may
forward the request on to that SSP using the procedures described in
the remainder of these steps.
6.1.2.3. LRF to LRF Routing
Communications between the LRF of two interconnecting SSPs may use
DNS or statically provisioned IP Addresses for reachability. Other
inputs to determine the path may be code-based routing, method-based
routing, Time of day, least cost and/or source-based routing.
6.1.3. The Signaling Path Border Element (SBE)
The purpose of signaling function is to perform routing of SIP
messages as well as optionally implement security and policies on SIP
messages, and to assist in discovery/exchange of parameters to be
used by the Media Function (MF). The signaling function performs the
routing of SIP messages. The SBE may be a B2BUA or it may act as a
SIP proxy. Optionally, an SF may perform additional functions such
as Session Admission Control, SIP Denial of Service protection, SIP
Topology Hiding, SIP header normalization, SIP security, privacy, and
encryption. The SF of an SBE can also process SDP payloads for media
information such as media type, bandwidth, and type of codec; then,
communicate this information to the media function.
6.1.3.1. Establishing a Trusted Relationship
Depending on the security needs and trust relationships between SSPs,
different security mechanisms can be used to establish SIP calls.
These are discussed in the following subsections.
6.1.3.2. IPSec
In certain deployments the use of IPSec between the signaling
functions of the originating and terminating domains can be used as a
security mechanism instead of TLS. However, such IPSec use should be
the subject of a future document as additional specification is
necessary to use IPSec properly and effectively.
6.1.3.3. Co-Location
In this scenario the SFs are co-located in a physically secure
location and/or are members of a segregated network. In this case
messages between the originating and terminating SSPs could be sent
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as clear text (unencrypted). However, even in these semi-trusted co-
location facilities, other security or access control mechanisms may
be appropriate, such as IP access control lists or other mechanisms.
6.1.3.4. Sending the SIP Request
Once a trust relationship between the peers is established, the
originating (or indirect) SSP sends the request.
6.2. Target SSP Procedures
This section describes the Target SSP Procedures.
6.2.1. TLS
The section defines the usage of TLS between two SSPs [RFC5246]
[RFC5746] [RFC5878]. When the receiving SSP receives a TLS client
hello, it responds with its certificate. The Target SSP certificate
should be valid and rooted in a well-known certificate authority.
The procedures to authenticate the SSP's originating domain are
specified in [RFC5922].
The SF of the Target SSP verifies that the Identity header is valid,
corresponds to the message, corresponds to the Identity-Info header,
and that the domain in the From header corresponds to one of the
domains in the TLS client certificate.
As noted above in Section 6.1.3.2, some deployments may utilize IPSec
rather than TLS.
6.2.2. Receive SIP Requests
Once a trust relationship is established, the Target SSP is prepared
to receive incoming SIP requests. For new requests (dialog forming
or not) the receiving SSP verifies if the target (request-URI) is a
domain that for which it is responsible. For these requests, there
should be no remaining Route header field values. For in-dialog
requests, the receiving SSP can verify that it corresponds to the
top-most Route header field value.
The receiving SSP may reject incoming requests due to local policy.
When a request is rejected because the originating (or indirect) SSP
is not authorized to peer, the receiving SSP should respond with a
403 response with the reason phrase "Unsupported Peer".
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Internet-Draft SPEERMINT Peering Architecture February 20116.3. Data Path Border Element (DBE)
The purpose of the DBE [RFC5486] is to perform media related
functions such as media transcoding and media security implementation
between two SSPs.
An example of this is to transform a voice payload from one codec
(e.g., G.711) to another (e.g., EvRC). Additionally, the MF may
perform media relaying, media security [RFC3711], privacy, and
encryption.
7. Address Space Considerations
Peering must occur in a common IP address space, which is defined by
the federation, which may be entirely on the public Internet, or some
private address space [RFC1918]. The origination or termination
networks may or may not entirely be in the same address space. If
they are not, then a network address translation (NAT) or similar may
be needed before the signaling or media is presented correctly to the
federation. The only requirement is that all associated entities
across the peering interface are reachable.
8. Acknowledgments
The working group would like to thank John Elwell, Otmar Lendl, Rohan
Mahy, Alexander Mayrhofer, Jim McEachern, Jean-Francois Mule,
Jonathan Rosenberg, and Dan Wing for their valuable contributions to
various versions of this document.
9. IANA Considerations
This memo includes no request to IANA.
10. Security Considerations
The level (or types) of security mechanisms implemented between
peering providers is in practice dependent upon on the underlying
physical security of SSP connections. This means, as noted in
Section 6.1.3.3, whether peering equipment is in a secure facility or
not may bear on other types of security mechanisms which may be
appropriate. Thus, if two SSPs peered across public Internet links,
they are likely to use IPSec or TLS since the link between the two
domains should be considered untrusted.
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Many detailed and highly relevant security requirements for SPEERMINT
have been documented in Section 5 of
[I-D.ietf-speermint-requirements]. As a result, that document should
be considered required reading.
Additional and important security considerations have been documented
separately in [I-D.ietf-speermint-voipthreats]. This document
describes the many relevant security threats to SPEERMINT, as well
the relevant countermeasures and security protections which are
recommended to combat any potential threats or other risks. This
includes a wide range of detailed threats in Section 2 of
[I-D.ietf-speermint-voipthreats]. It also includes key requirements
in Section 3.1 of [I-D.ietf-speermint-voipthreats], such as the
requirement for the LUF and LRF to support mutual authentication for
queries, among other requirements which are related to
[I-D.ietf-speermint-requirements]. Section 3.2 of
[I-D.ietf-speermint-voipthreats] explains how to meet these security
requirements, and then Section 4 explores a wide range of suggested
countermeasures.
11. Contributors
Mike Hammer
Cisco Systems
Herndon, VA - USA
Email: mhammer@cisco.com
--------------------------------------------------------------
Hadriel Kaplan
Acme Packet
Burlington, MA - USA
Email: hkaplan@acmepacket.com
--------------------------------------------------------------
Sohel Khan, Ph.D.
Comcast Cable
Philadelphia, PA - USA
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o 19: Additional change to the IPSec section at Jari Arkko's
request.
o 18: Made several changes based on feedback from Adrian Farrel,
Bert Wijnen, Dan Romascanu, Avshalom Houri, Russ Housley, Sean
Turner, Tim Polk, and Russ Mundy during IESG review.
o 17: Misc. updates at the request of Gonzalo, the RAI AD, in order
to clear his review and move to the IESG. This included adding
terminology from RFC 5486 and expanding the document name.
o 16: Yes, one final outdated reference to fix.
o 15: Doh! Uploaded the wrong doc to create -14. Trying again. :-)
o 14: WGLC ended. Ran final nits check prior to sending proto to
the AD and sending the doc to the IESG. Found a few very minor
nits, such as capitalization and replacement of an obsoleted RFC,
which were corrected per nits tool recommendation. The -14 now
moves to the AD and the IESG.
o 13: Closed out all remaining tickets, resolved all editorial
notes.
o 12: Closed out several open issues. Properly XML-ized all
references. Updated contributors list.
o 11: Quick update to refresh the I-D since it expired, and cleaned
up some of the XML for references. A real revision is coming
soon.
13. References13.1. Normative References
[I-D.ietf-speermint-requirements]
Mule, J., "Requirements for SIP-based Session Peering",
draft-ietf-speermint-requirements-10 (work in progress),
October 2010.
[I-D.ietf-speermint-voipthreats]
Seedorf, J., Niccolini, S., Chen, E., and H. Scholz,
"Session Peering for Multimedia Interconnect (SPEERMINT)
Security Threats and Suggested Countermeasures",
draft-ietf-speermint-voipthreats-07 (work in progress),
January 2011.
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